The present invention relates generally to the field of digital imaging devices, such as digital x-ray imaging systems. More particularly, the invention relates to a technique for testing portions of driver circuitry in digital detector arrays of the type used in direct digital x-ray systems.
Increasing emphasis is being placed in a wide variety of imaging devices on direct digital imaging techniques. In x-ray systems, for example, techniques are being developed for detecting intensities of radiation striking a digital detector surface during examinations. In the course of the examinations, an x-ray source emits radiation which traverses a subject, such as a human patient. The x-ray intensity is, however, affected by the various tissues and structures within the subject, resulting in intensity variations at the detector, which is placed behind the subject with respect to the source. By identifying intensities of resulting radiation at a large number of locations arranged in a matrix, the detector allows a data set to be acquired which can be used to reconstruct a useful image of the subject.
In digital detectors for x-ray systems a detector surface is divided into a matrix of picture elements or pixels, with rows and columns of pixels being organized adjacent to one another to form the overall image area. When the detector is exposed to radiation, photons impact a scintillator coextensive with the image area. A series of detector elements are formed at row and column crossing points, each crossing point corresponding to a pixel making up the image matrix. In one type of detector, each element consists of a photodiode and a thin film transistor. The cathode of the diode is connected to the source of the transistor, and the anodes of all diodes are connected to a negative bias voltage. The gates of the transistors in a row are connected together and the row electrode is connected to scanning electronics. The drains of the transistors in each column are connected together and each column electrode is connected to additional readout electronics. Sequential scanning of the rows and columns permits the system to acquire the entire array or matrix of signals for subsequent signal processing and display.
In use, the signals generated at the pixel locations of the detector are sampled and digitized. The digital values are transmitted to processing circuitry where they are filtered, scaled, and further processed to produce the image data set. The data set may then be used to store the resulting image, to display the image, such as on a computer monitor, to transfer the image to conventional photographic film, and so forth. In the medical imaging field, such images are used by attending physicians and radiologists in evaluating the physical conditions of a patient and diagnosing disease and trauma.
In digital detectors of the type described above, problems may arise due to short circuits which may exist or develop between elements of the detector circuitry. In particular, both during the manufacturing and assembly steps employed in producing the detectors and related circuitry, electrical shorts may occur between conductors of adjacent rows, between rows and bias supplies, and between rows and ground. Such shorts may lead to imaging defects which significantly impair the utility of the detector. In certain cases, early detection of such defects may permit replacement or repair of detector circuitry, or may indicate the need to replace an entire section of the detector circuitry.
While such short circuits can be detected by various procedures, conventional techniques are time consuming and difficult to implement. There remains a need, therefore, for efficient, rapid, and dependable techniques designed to detect short circuits and similar defects in scan driver circuitry. There is a particular need for a technique which can be implemented in direct digital detector circuits, such as those used in digital x-ray imaging systems.
The present invention provides a technique for identifying potential short circuits and other defects in scan driver circuitry designed to respond to these needs. The technique is particularly well suited for implementation in circuitry associated with digital detectors comprising an array of rows and columns, and capable of producing signals representative of radiation impacting a plurality of pixels defined by the rows and columns. The structure employed in the technique is designed to facilitate its implementation in one or more row driver integrated circuits. The technique advantageously employs an arrangement of transistors which are dedicated to testing for shorts in row output drivers, thereby reducing or eliminating the need to externally probe outputs simultaneously while individually enabling each row of the detector. Moreover, open-test outputs may be electrically linked in the structure, such as via wired-NOR functional groups. This further minimizes the number of inputs to be monitored by subsystem control electronics.